Abstract: Accompany with adoption of cable in medium voltage distribution network,the level of capa-citance current of the system increases.More and more researchers focus on how to measure the capacitance current to the ground of the distribution network system precisely and swiftly.The paper designed a measurement apparatus of capacitance current based on signal-injection method,which made use of direct digi-tal frequency synthetics(DDFS) technology to produce various frequency signal of current and made use of Fourier to measure the capacitance current of neutral non-valid grounded system.It has advantages of high precision,convenient and flexible for use.